[Ifeffit] switching to FEFF8 in Artemis

[Bruce Ravel]

Hi Matthew,

Your questions never got answered!  Yikes!


> I have FEFF8 and would like to get Artemis to use it for EXAFS
> calculations.  I figured out how to use the Edit Preferences menu to
> point Artemis to the executable and how to get the ATOMS module to
> use the FEFF8_EXAFS template to generate feff.inp files which result
> in the generation of feffNNNN.dat files. What doesn't work is that
> QFS still uses the feff6 template.  Without doing something nasty
> like overwriting the feff6 template with a copy of the feff8
> template, how can I get the whole system to work with feff8?

Is "QFS" the quick fist shell theory thingie in the Theory menu?  You
are correct -- it does not give you a choice.  Using feff6 is
hardwired into it.  It seems silly to me to use feff8 for the quick
first shell thing, but for the sake of completeness, it should be
fixed.

Your other option is to run feff8 outside of artemis and import the
results.  When you "Open file", you can point the file browser at a
feff.inp file.  That will load the feff.inp file, notice that there
are feffNNNN.dat files, and ask how many you want to import.

That's the work-around.  I would like to understand your problem a bit
better so I can improve it.

> Also, I found that when simulating Pu3Ga (a simple Cu3Au structure) out to
> 10A, feff8 chucked up an error about too many paths, while feff6 did it
> happily.  Is there a
> way to fix that?

One of these might help:

http://leonardo.phys.washington.edu/feff/wiki/index.php?title=CRITERIA

http://leonardo.phys.washington.edu/feff/wiki/index.php?title=PCRITERIA

> How much difference does using feff8 make in terms of proper description of
> MS paths for k>2, anyway?  Do I really win by go ing through the trouble?

Were the question about k>4, I would say with confidence that you do
not win.  For k>2, I suspect the answer is "not much".  For a highly
asymmetric material, feff8 should significantly improve the
description of the potential surface by virtue of moving charge around
self-consistently.  But the potentials are still spherical, so there
really aren't bonds in feff8.  As you move away from the potential
surface by increasing energy, you become increasingly insensitive to
those details.

Is k=2 "far enough" from the potential surface?  I don't know.  There
are a lot of issues down there.  k=2 is a region where the details of
the background removal are highly correlated with the fit results.  Do
you need a better feff or a better background removal?  It seems like
a tough question to answer generally.

HTH,
B



-- 
 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/